Circularly polarized radio frequency antenna

ABSTRACT

A radio frequency antenna including an antenna element having an open-ended waveguide section coupled to a first feed means for establishing radio frequency energy having linear polarization, the electric field vector of such energy being disposed normal to a pair of opposing wall portions of the waveguide. The antenna element includes a microwave circuit means for establishing radio frequency energy having a linear polarization orthogonal to the polarization of the first mentioned radio frequency energy. The microwave circuit includes a dielectric and a conductive sheet disposed over the dielectric; such sheet providing one of the wall portions of the waveguide section. The conductive sheet has an array of notches formed therein, such notches being disposed adjacent the open end of the waveguide section. A second feed means is coupled to the array of notches for establishing radio frequency energy having a linear polarization, the electric field thereof being in the plane of the dielectric and hence normal to the polarization established by the first feed means. With such arrangement, the first and second feed means are fed in quadrature and the phase centers of the orthogonally disposed polarized radio frequency energy at the open end of the waveguide are substantially coincident to thereby enable the antenna to establish circularly polarized radio frequency energy in free space. Further, the radio frequency energy fed to the second feed means is distributed to an array of notch shaped elements thereby increasing the power handling capability of the antenna element. Still further, the antenna element is relatively compact and suitable to be arranged in relatively wide scan angle coverage.

BACKGROUND OF THE INVENTION

This invention relates generally to radio frequency antenna and more particularly to radio frequency antenna adapted to operate with radio frequency energy having any one of a variety of polarizations.

As is known in the art, it is frequently desirable to use an antenna element which may operate with any one of a variety of polarization (i.e. linear or circular). One type of antenna element capable of such operation is sometimes referred to as a "double ridged" horn. One antenna element of such type generally would include a vertical feed and an independent horizontal feed, the phase centers of such feeds being coincident. For circular polarization the two feeds are fed with radio frequency energy having a quadrature phase difference. In order to provide efficient matching to free space over a relatively wide frequency band, say in the order of 3.5 to 1, it is generally required that the width of the horn be larger than half the wavelength at the nominal operating frequency of the antenna and sometimes be as large as one wavelength. In an array antenna, a plurality of antenna elements are provided in order to attain a relatively wide scan angle, say in the order of 120 degrees. In such array, it is generally required that the phase centers of adjacent ones of the plurality of antenna elements be displaced by less than one half wavelength. It follows then that while a double ridged horn antenna may be adapted to operate with radio frequency energy having circular polarization, such an antenna element may not be readily used, because of its size, in an array antenna having relatively wide scan angles.

In another type of array antenna adapted to provide a variety of polarization each one of the antenna elements includes an orthogonally disposed pair of printed circuit notch shaped antenna elements. One such type of antenna is described in U.S. Pat. No. 3,836,976 entitled "Closely Spaced Orthogonal Dipole Array," inventors George J. Monser, George S. Hardie, John R. Ehrhardt and Terry M. Smith, issued Sept. 17, 1974, and assigned to the same assignee as the present invention. While such antenna is adapted to operate with circularly polarized radio frequency energy over relatively wide scan angles and over a relatively wide band of frequencies, such antenna is limited in its power handling capability and hence is not suitable for use in those applications where such antenna is fed by a transmitter adapted to transmit relatively large amounts of power.

SUMMARY OF THE INVENTION

In accordance with the present invention an antenna element is provided having an open ended waveguide section coupled to a first feed means for establishing radio frequency energy having linear polarization, the electric field vector of such energy being disposed normal to a pair of opposing wall portions of the waveguide. The antenna element includes a microwave circuit means for establishing radio frequency energy having a linear polarization orthogonal to the polarization of the first mentioned radio frequency energy. The microwave circuit includes a dielectric and a conductive sheet disposed over the dielectric; such sheet providing one of the wall portions of the waveguide section. The conductive sheet has an array of notches formed therein, such notches being disposed adjacent the open end of the waveguide section. A second feed means is coupled to the array of notches for establishing radio frequency energy having a linear polarization, the electric field thereof being in the plane of the dielectric and hence normal to the polarization established by the first feed means.

With such arrangement, the first and second feed means are fed in quadrature and the phase centers of the orthogonally disposed polarized radio frequency energy at the open end of the waveguide are substantially coincident to thereby enable the antenna to establish circularly polarized radio frequency energy in free space. Further, the radio frequency energy fed to the second feed means is distributed to an array of notch shaped elements thereby increasing the power handling capability of the antenna element. Still further, the antenna element is relatively compact and suitable to be arranged in an array for providing relatively wide scan angle coverage.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the invention itself, may be more fully understood from the description read together with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a radio frequency antenna system including an array of antenna elements according to the invention;

FIG. 2 is a perspective view of the array of antenna elements used in the antenna system of FIG. 1;

FIG. 3 is a top plan view of a member used to form a portion of one of the antenna elements of FIG. 2;

FIG. 4 is a bottom plan view of the member of FIG. 3;

FIG. 5 is a side elevation view of the member of FIG. 3;

FIG. 6 is an end elevation view of the member of FIG. 3;

FIG. 7 is an exploded, isometric view of a strip transmission line feed network used to form a portion of one of the antenna elements of FIG. 2;

FIG. 8 is a plan view of the member of FIG. 3 and the strip transmission line feed network of FIG. 7 disposed thereon;

FIG. 9 is an exploded isometric drawing, partly broken away, of one of the antenna elements in the array of FIG. 2;

FIG. 10 is a perspective view of the bottom portion of the member of FIG. 3;

FIG. 11 is an exploded cross-sectional side elevation view of a pair of members of FIG. 3 and a pair of strip transmission line networks of FIG. 7, such pair of feed networks, and pair of members forming the antenna element of FIG. 9, the cross-section of one of such members being taken along lines 11--11 of FIG. 3, one of such pair of members being the member shown in FIG. 3;

FIG. 12 is a cross-sectional side elevation view of the antenna element of FIG. 11; and

FIG. 13 is a cross-sectional view showing a portion of a feed probe used to feed the portion of the antenna element of FIG. 12 formed by the pair of members and also showing a portion of such pair of members, such FIG. 13 being of region 13--13 of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 a multibeam radio frequency antenna system 10 adapted to operate over a relatively wide band of frequencies, here 4.8 GHZ to 18.0 GHZ is shown to include a radio frequency lens 12 having a plurality of feed ports 14a-14n disposed along a portion of the periphery of such lens 12 and a plurality of here eight array ports 16₁ -16₈ disposed along an opposing portion of the periphery of the lens 12, the plurality of array ports 16₁ -16₈ being coupled to an array 20 of a plurality of, here eight, identically constructed antenna elements 22₁ -22₈ through a power distribution network 24 the details of which will be described hereinafter. Sufficient to say here, however, that the shape of the lens 12, the construction of the power distribution network 24 and the arrangement of the antenna elements 22₁ -22₈ are selected such that n collimated beams of radio frequency energy are formed in free space by the antenna system 10, each one of such n beams having a different direction and each one of such n beams having circularly polarized radio frequency energy.

Referring to FIG. 2 array 20 is shown to include a plurality of identically constructed conductive members 26₁ -26₁₀, an exemplary one thereof, here member 26₁ being shown in detail in FIGS. 3-6 and 10 and a plurality of, here nine, identically constructed microwave circuits, here strip transmission line circuits 27₁ -27₉, an exemplary one thereof, here circuit 27₁ being shown in detail in FIG. 7. A pair of members 26₁ -26₁₀ and a pair of circuits 27₁ -27₉ form one of the identically constructed antenna elements 22₁ -22₈. Thus, as shown FIGS. 9, 11 and 12 an exemplary one of the antenna elements 22₁ -22₈, here antenna element 22₁ is shown to include conductive members 26₁ and 26₂ and strip transmission line circuits 27₁ and 27₂. (It is noted that constituent parts of member 26₁ and circuit 27₁ are designated by a subscript 1 and constituent parts of member 26₂ and circuit 27₂ are designated by a subscript 2). More particularly, as shown in FIGS. 9, 11 and 12, the upper surface of the antenna 22₁, is formed, in the frontal portion thereof, by the bottom surface 28₁ of strip transmission line circuit 27₁ and in the rearward portion thereof by the bottom surface 30₁ of conductive member 26₁ ; whereas the lower surface of antenna elements 22₁ is formed, in the frontal portion thereof, by the upper surface 32₂ of strip transmission line circuit 27₂ and, in the rearward portion thereof, by the upper surface 34₂ of conductive member 26₂.

Referring now to FIG. 7 an exemplary one of the identically constructed strip transmission line circuit 27₁ -27₉, here strip transmission line circuit 27₁ is shown in detail to include a pair of dielectric support structures 40₁, 42₁ of any suitable material, here Teflon Fiberglas material having a dielectric constant of 2.56. Initially, each one of the dielectric support structures 40₁, 42₁ has a sheet of conductive material, here copper clad on the upper and lower surfaces thereof. The sheet of conductive material on the lower surface of dielectric support structure 42₁ is removed entirely with a suitable chemical etchant whereas a plurality of, here four, flared notches 44₁ are etched into the conductive material 32₁ clad onto the upper surface of such dielectric support structure 40₁ using conventional photolithographic-chemical etching techniques. Each one of the notches 44₁ has a narrow portion 46₁ and a wider portion 48₁. The notches 44 are separated from each other a distance less than a half wavelength at the smallest operating wavelength of the antenna. More particularly, here the center-to-center spacing of the notches 44 is 0.350 inches. The width of the wide portion 48 is here 0.260 inches and the widths of the narrow portion 46 is here 0.050 inches. The length of the wide portion 48 is here 0.130 inches and the length of the narrow portion 46 is here 0.842 inches. Considering now the second one of the pair of dielectric support structure 42₁ a similar pattern of four flared notches 50₁ is etched into the conductive sheet 28₁ clad to the bottom surface of such dielectric support structure 42₁. Each one of the slots 50₁ is identical to the slots 44₁ formed on the conductive sheet 32₁ clad to the upper surface of dielectric support 40₁. The conductive sheet clad to the upper surface of the dielectric support structure 42₁ is etched to form a feed network 52₁. The feed network 52₁ is a strip transmission line circuit having strip conductor 54₁ disposed between a pair of ground plane conductors formed by the conductive sheets 28₁, 32₁, and separated from such sheets 28₁, 32₁ by the dielectric support structures 40₁, 42₁. The feed network 52₁ includes a first two-to-one power divider section 56₁ the output of which into turn feeds a pair of two-to-one power divider sections 58₁, 60₁. Each one of the three power divider sections 56₁, 58₁, 60₁ includes a step-matching transformer section 62₁. Thus, power fed to the strip transmission line feed network 52₁ is divided equally, and in phase, to each one of four feed lines 64₁. Each feed line 64₁ is disposed underneath the narrow portions 46₁ of a pair of notches 48₁, 50₁ as shown in FIG. 8, the notches 44₁ on conductive sheet 32₁ being in registration with the notches 50₁ formed in conductive sheet 28₁. When strip transmission line feed network 54₁ is fed radio frequency energy from a coaxial connector 66₁ (FIG. 8) having a center conductor 68₁ electrically connected to a strip conductor 54₁ of the strip transmission line feed network 52₁, 54₁ and outer conductors 70₁ electrically connected to conductive sheets 32₁, 28₁, the strip transmission line circuit 27₁ couples energy to the feed lines 64₁ and then to notches 44₁ and 50₁ whereupon such feed energy is then radiated into free space with an electric field vector disposed in the plane of the strip transmission line circuit 27₁ as shown by the vector E₁ in FIGS. 1 and 8. Thus, the energy radiated by the notches 44₁, 50₁ is linearly polarized; more particularly, here horizontally polarized.

Referring now in more detail to members 26₁ -26₁₀, each one of such members 26₁ -26₁₀ is constructed from a block of electrically conductive material, here aluminum, here having outer dimensions of 4.037 inches (length) and 2.250 inches (width). Such block has machined therein S-shaped side wall portions 74₁, 76₁ (FIG. 3) and a rear wall portion 78₁ having a recess notch or 80₁ formed therein. The depth of the side wall and rear wall portions is here substantially 0.325 inches. Also machined into the upper surface 34₁ of the member 36₁ is a tapered ridge 82₁, as shown, here having a width of 0.20 inches. The tapered ridge 82₁ has an aperture 84₁ formed in the upper, flat top portion 86₁ thereof, the flat top portion 86₁ terminating in a tapered portion 88₁, (FIGS. 3, 9) as shown. The length from the end of the tapered ridge 82₁ to the end of the member 36 is here 2.2 inches. The length of the tapered portion 88₁ is here 0.9 inches. The depth of the notch 80₁ formed in the rear wall portion 78₁ is here 0.075 inches, such notch 80₁ having a length along the rear wall portion 78₁ of, here, 0.588 inches. It is noted that the separation between the side wall portions 74₁, 76₁ disposed laterally to the tapered portion 88₁ is relatively constant, here 1.5 inches; however, such separation decreases, here along curved paths, 90₁, 92₁, as such rear wall portions 74₁, 76₁ extend towards the rear wall portion 78₁. As will be discussed in more detail hereinafter, the converging of the side wall portions 74₁, 76₁ as they extend towards the rear wall portion 78₁ in the region behind the aperture 84₁ (such aperture being the area where the antenna element 22₁ formed by such member 26₁ together with member 26₂ is fed by the coaxial connector in a manner to be described) improves the impedance matching between the coaxial connector and the antenna element 22₁. Member 26₁ also has holes 100₁ drilled through it, such being used for bolting the members together with bolts and screws 107₁ as shown in FIG. 2. A hole 108₁ is formed partly into the upper surface of the member 26₁ and is used to receive an alignment pin 109₂ (FIGS. 4-6) formed on the bottom surface of member 26₂, as shown also in FIG. 10. Disposed along the curved regions 90₁, 92₁ (FIG. 3) of the side wall portions 74₁, 76₁ are open ended channels 110₁, 112₁. Channels 110₁, 112₁ are here formed of curved conductive strips 114₁, 116₁ here aluminum, having ends 118₁, 120₁ spaced from, and affixed to, side walls 74₁, 76₁ respectively. The spacing is provided by aluminum spacers 122₁, 124₁ such ends 118₁, 120₁ and spacers 122₁, 124₁ being affixed to the side wall portions 74₁, 76₁ through any convenient means as by bolts or a suitable electrically conductive epoxy, not shown. The spacers 122₁, 124₁ here have a thickness of 0.1 inches and the length of the channels 110₁, 112₁ is here 0.6 inches. The channels 110₁, 112₁, are effective in removing unwanted surface currents produced along the side wall portions 74₁, 76₁ such currents being associated with radio frequency energy having a frequency of here 13.2 GHZ. That is, it was noted that there was a significant loss of gain the antenna at about 13.2 GHZ. It was also noticed that the channels 110₁, 112₁ removed the loss of gain in the region of 13.2 GHZ. It is noted that the length of the channels 110₁, 112₁ is here about three quarters of the wavelength of the frequency associated with the unwanted surface currents.

The side wall portion 74₁, 76₁ (FIG. 3) disposed between the tapered ridge 82₁ and the frontal end of the member 26₁ are flared outwardly along a nonlinear path 131₁ to increase the surface length of the side wall portions 74₁, 76₁ from the tapered ridge 82₁ to free space within the fixed longitudinal length of the antenna element 26₁ thereby providing a relatively compact antenna element with a side wall length sufficiently long to provide an adequate transition region between the tapered ridge and free space.

Referring now to the bottom surface 30₁ of member 26₁ (shown more clearly in FIGS. 4, 5, 6 and 10) such surface 30₁ also has a tapered ridge 126₁ formed thereon; here, however the flat portion 123₁ of the ridge 126₁ has a turret shaped conductive post 122₁ (here shown) press fit therein by a pin shaped end 127₁ as shown in FIG. 13. Post 122₁ has a hole 128₁ drilled therein as shown for receiving the center conductor 142₁ of a coaxial connector 140₁ (FIG. 11) in a manner to be described in detail in connection with FIG. 13. It is noted from FIG. 5 that the tapered ridges 82₁, 126₁ formed on the upper and lower surfaces of member 26₁ are in alignment or registration with each other. Further, it is evident that the post 122₁ of member 26₁ fits into the aperture 84₂ of member 26₂ as shown in FIGS. 9, 11 and 12.

When members 26₁, 26₂ and strip transmission line circuits 27₁, 27₂ are affixed together (here by screws 107 (FIG. 2) and conductive epoxy, not shown, disposed on the portions of the copper conductive sheets 28, 32 of circuits 27₁ -27₉ (FIGS. 9 and 10) which contact portions of the conductive members 26₁ -26₁₀), the lower surface 30₁ of member 26₁, and the upper surface 34₂ of member 26₂ form opposing upper and lower wide surfaces of the rear portions of a hollow rectangular, open ended waveguide structure; the bottom ground plane conductor 28₁ of circuit 27₁ and the upper ground plane conductor 32₂ of circuit 27₂ form opposing upper and lower wide wall portions of the forward portion of the rectangular waveguide structure and side wall portions 74₂, 76₂ and rear wall portion 78₂ form narrow side and rear walls of such open ended, rectangular waveguide. More particularly, the affixed members 26₁, 26₂ formed a tapered ridge rectangular waveguide antenna element 22₁. Surfaces 145₁ of member 26₁ contact surfaces 149₂ of member 26₂ as shown in FIG. 11. The side and back edges of the circuits 27₁ -27₉ are covered with a conductive epoxy (not shown) to electrically connect the side and back edges of the conductive sheets 28, 32. Further, circuits 27₁, 27₂ (FIG. 3) fit into a groove 146₁, 148₁, 152₁ formed in the conductive members 26₁ -26₈ so that the flat portions 86₂, 123₁ of the ridges 126₂, 82₁ are separated a distance "d" (FIG. 13), and the wide walls of the waveguide, i.e. conductive sheets 28₁, 32₂, are separated a distance "b" (FIG. 12). The distances "b" and "d" are designed so that the waveguide propagates even in the TE₁₀ mode. Here "d" is 0.045 inches and "b" is 0.325 inches. The tapered ridge waveguide antenna elements 22₁ -22₈ are fed by the coaxial transmission line through coaxial connectors 140₁ -140₈ (FIGS. 1, 9, 11 and 12) having a center conductor 142₁ (FIG. 13) passing through hole 144₁ (FIGS. 5, 11, 13) and the end of such center conductor 142₁ press fit to post 122₁ to provide electrical and mechanical contact to post 122₁. The outer conductor 145₁ is electrically and mechanically connected to the member 26₂ through screws 141₁ as shown in FIGS. 9 and 11. The inner conductor 142₁ is separated from the walls of the hole 144₁ by a dielectric sleeve 143₁ as shown in FIG. 13. A ferrite ring 147₁ is disposed around the inner conductor 142₁ between the dielectric 143₁ and the post 126₁, as shown in FIG. 13 to provide impedance matching between the coaxial connector 142₁ and the post 122₁. Radio frequency energy fed to the antenna element 26₁ via connector 140₁ thus launches radio frequency energy into cavity 148₁ (FIG. 12), such energy travelling towards the open end 160₁ of the cavity in the TE₁₀ mode having an electric field vector extending between the wide surfaces of the waveguide as shown by arrow E₂ in FIGS. 1 and 12.

It is noted then that each one of the antenna elements 22₁ -22₈ is adapted to transmit radio frequency energy having vertical polarization, (as when feeding radio frequency energy to connector 140₁ -140₈ or horizontally polarized radio frequency energy (as when feeding radio frequency energy to connector 66₁ -66₉). Further, if radio frequency energy is fed equally to both connectors 140₁ -140₈ and 66₁ -66₉ and the phase of the energy fed to such connectors differs by ninety degrees, such antenna element will transmit circularly polarized radio frequency energy in free space.

Thus, referring again to FIG. 1 the details of the power distribution network 24 will now be described. As mentioned above in connection with FIG. 1 the array ports 16₁ -16₈ are coupled to the array 20 through a power distribution network 24. The power distribution network 24 includes a plurality of, here eight, hybrid couplers, 150₁ -150₈ having a pair of input terminals 152a₁, 152b₁ -152a₈, 152b₈ and a pair of output terminals 154a₁, 154b₁ -154a₈, 154b₈. One of the input terminals 152a₁ -152a₈ is coupled to a corresponding one of the array ports 16₁ -16₈, as shown and the other one of such input 152b₁ -152b₈ terminals is terminated in a matched load 156₁ -156₈ as shown. Hence the power fed to the couplers 150₁ -150₈ is divided equally between output ports 154a₁, 154b₁, through 154a₈, 154b₈ respectively, but the signals at such output ports differ in phase from one another by ninety degrees. That is, considering coupler 150₁ for example, the signals at output ports 154a₁, 154b₁ differ in phase by ninety degrees. The signals produced at ports 154a₁ -154a₈ are fed to two-to-one power dividers 160₁ -160₈, respectively as shown. The power in the signals fed to dividers 160₁ -160₈ is divided equally and in phase. The pair of signals produced at the outputs of dividers 160₁ -160₈ are fed to two-to-one power combiners 162₁ -162a₁ as shown. It is noted that one of the inputs of combiners 162₁, 162₉ are terminated in matched loads 164, 166. The outputs of power combiners 162₁ -162₉ are fed to coaxial connectors 66₁ -66₉ which feed strip transmission line circuits 27₁ -27₉. Thus, if the radio frequency signal phases at output ports 154 a₁ -154a₈ are represented as: φ, 2φ, . . . 8φ respectively, the signal phases at connectors 66₁ -66₉ may be represented as: φ/2, 3φ/2, 5φ/2 . . . 15φ/2, 17φ/2, respectively and the signal phases at connectors 140₁ -140₈ may be represented as: φ+(π/2) . . . 8φ+(π/2) respectively.

Considering first an exemplary pair of the pair of inner connectors 66₂, 66₃ through 66₇, 66₈, here for example the pair of connectors 66₆, 66₇ it is noted that such connectors feed the strip transmission line circuits 27₆, 27₈ which form a portion of the upper and lower wide walls of the antenna element 22₆. Further, the signal phases fed to such terminals 27₆, 27₈ may be represented as: 11φ/2 and 13φ/2 respectively. Therefore, in the region between the circuits 27₆, 27₈ at the open end of the antenna element 26₆, the signals combine so that the resulting signal phase may be represented as: 6φ in the region between the circuits 27₆, 27₈. Further, such signal has a horizontal polarization. Still further, the signal phase fed to connector 140₆ of such antenna element 22₆ may be represented as: 6φ+(π/2), has a vertical polarization, and has a phase center coincident with the phase center of the horizontally polarized energy of the signal produced by the pair of strip transmission line circuits 27₆, 27₈. Thus, the energy associated with antenna element 22₆, in free space, is circularly polarized. It is noted that the above discussion applies to antenna elements 22₂ -22₇, however there is some distortion with the antenna elements 22₁, 22₈ which are at the end of the array 20. However, with a large array, i.e. an array having 16 elements or more the effect of the end elements is minimized.

Having described a preferred embodiment of the invention it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. It is believed therefore that this invention should not be restricted to the disclosed embodiment but rather should be limited only by the spirit and scope of the appended claims. 

What is claimed is:
 1. A radio frequency antenna for producing circularly polarized radio frequency energy comprising:(a) a waveguide section having a pair of opposing walls; (b) a first feed means for establishing radio frequency energy in such waveguide section having a linear polarization with an electric field disposed normal to the pair of opposing walls of the waveguide; (c) a first microwave circuit means for establishing radio frequency energy having a linear polarization disposed normal to the electric field of the first mentioned linear polarization, such microwave circuit means comprising: a dielectric; a strip conductor circuit disposed over a first surface of the dielectric; and a ground plane conductor disposed over a second opposite surface of the dielectric, such ground plane providing one of the pair of opposing walls of the waveguide; (d) a second microwave circuit means for establishing radio frequency energy having a linear polarization disposed normal to the electric field of the first mentioned linear polarization, such microwave circuit means comprising: a second dielectric; a second strip conductor circuit disposed over a first surface of the second dielectric; and a second ground plane conductor disposed over a second surface of the second dielectric, such second ground plane conductor providing the second one of the pair of opposing walls of the waveguide section; and, (e) a power distribution means for coupling radio frequency energy to the first and second microwave circuit means with equal power and equal phase shift and for coupling radio frequency energy to the first feed means with a phase shift 90° with respect to the phase shift of the energy fed to the first and second microwave circuit means.
 2. The antenna recited in claim 1 wherein the ground plane conductor of the one of the microwave circuit means includes a plurality of notch radiating elements formed therein and a feed network for distributing the microwave energy fed thereto between each one of the plurality of notch radiating elements. 